This invention relates to a fused solid electrolyte capacitor package and more particularly to such a package wherein the fuse is an exothermically alloyable bimetal member.
Electrolytic valve metal capacitors are most commonly employed as AC filters being connected across a low impedance DC power bus. When a defect occurs in the dielectric film of the solid electrolyte capacitor, abnormally large currents tend to flow which may heat the porous valve metal anode to very high temperatures. Fault currents flowing in a shorted solid electrolyte tantalum capacitor frequently raise the temperature of the tantalum anode to a self sustaining combustion temperature which in turn may cause violent and severe damage to neighboring electronic components.
The problem of fusing a solid electrolyte capacitor is particularly difficult to solve. It has been found that wire fuses made of an elemental metal or of an alloyed metal may be employed in a solid electrolyte capacitor package and may work satisfactorily only when the fault current is due to a thoroughly shorted capacitor and is thus very high. But it has also been found that when moderate fault currents are present, a high melting metal fuse, e.g. nickel melting at 1400.degree. C, may become hot long enough prior to melting so as to itself cause the capacitor body to ignite.
On the other hand, a medium temperature melting fuse wire (e.g. a cadmium-silver alloy melting at about 350.degree. C) will melt at moderate fault currents but will remain confined as a continuous molten strand within the organic encapsulant cavity so that it does not open the circuit. A cavity in the housing may be provided adjacent the low melting fuse to overcome the above-mentioned confinement problem, only with additional manufacturing difficulty and higher costs.
It is accordingly an object of this invention to provide a simple fail-safe fusing feature in a solid electrolyte capacitor package.